ABOARD A U.S. MILITARY AIRCRAFT, Feb 10 (Reuters) - The topU.S. military officer said on Sunday he favored the idea ofarming Syrian rebels during discussions within the Obamaadministration about how to help resolve the country's civilwar, but there was never a specific plan under review.

General Martin Dempsey, chairman of the Joint Chiefs ofStaff, said he thought arming the rebels might help end thecrisis more quickly and avert the collapse of governmentinstitutions, which could lead Syria to become a failed state.

"Conceptually I thought if there were a way to resolve themilitary situation more quickly it would work to the benefit notonly of the Syrian people but also us," Dempsey told reportersaboard his plane from Afghanistan, where he attended achange-of-command ceremony for NATO-led forces.

"A failed state is defined by the collapse of itsinstitutions," he said. "And so conceptually we thought aboutways to prevent that from happening. Conceptually I was inagreement. Now there were enormous complexities involved that westill haven't resolved."

Dempsey and Defense Secretary Leon Panetta acknowledgedtheir support for arming Syrian rebels during a Senate hearinglast week, the first time they had been queried about the issue.

The CIA and State Department had recommended arming therebels last year, but President Barack Obama ultimately decidedagainst pursuing the option.

The revelation that Dempsey and Panetta had supported theidea along with CIA and State Department officials underlinedthe divisions in the administration over how to deal withthe Syrian crisis.

IDENTIFYING THE OPPOSITION

Lawmakers including Senator John McCain have been criticalof the administration's decision against taking more aggressiveaction.

So far the administration has limited support to providingnon-lethal aid for rebels in the 22-month-old conflict, whichhas killed an estimated 60,000 people.

A disparate group of rebel factions are fighting tooverthrow Syrian President Bashar al-Assad, whose troops arewell-trained and armed. U.S. officials still have not been ableto get a clear picture of all the groups and their ideologicalleanings.

"We still have a challenge identifying who among theopposition, if they achieved a position of dominance, wouldcommit themselves to the longer-term objectives of establishinga representative government, an end to violence, preservation ofthe institutions so that Syria doesn't become a failed state,"he said.

Dempsey said he did not believe that the possibility ofarming Syrian rebels had been completely ruled out by the Obamaadministration.

"No one has taken any option off the table in anyconversation in which I've been involved," he said.

Dempsey declined to say whether he still supported armingthe rebels.

"What I will say is of the options that we have beenconsidering, they all hinge on a much clearer understanding ofthe environment than I believe we currently have."

Dempsey said the United States was continuing to work withIsrael, Turkey and Jordan to address common interests andconcerns related to the Syrian conflict, from proliferation ofballistic missiles and humanitarian issues to chemical andbiological weapons.

He said the United States still believes Syrian chemicalweapons are secure.

"I want to be clear that we don't have persistent or perfectvisibility. On the occasions when we have noted movement,they've been movements that appear to us to be intended tosecure them, not to use them. But our ability to have acompletely clear understanding is somewhat limited."